At the scale of an electronic package, electronic signals with frequencies above about one (1) gigahertz (GHz) range tend to behave more like electromagnetic waves as opposed to current flowing through conductors. Such wave behavior includes the tendency to travel through free space and dielectric materials, causing unwanted cross talk between conductors and the emission of electromagnetic interference (EMI). In general, higher operating frequencies result in higher levels of cross talk and radio frequency interference emissions.
For applications with operating frequencies above about one GHz, the geometry of the conductors in an electronic package becomes critical to its function. Conductors must often be shaped into impedance-matched transmission line structures to reduce unwanted signal reflections. Shielding is required around nearly every signal conductor in order to eliminate unwanted cross talk with other signal conductors. Finally, the entire electronic package, including its interconnections to the larger system must be substantially shielded to prevent electromagnetic waves from escaping as unwanted EMI. This is accomplished by wrapping the entire electric package or subsystem in a ground conductor.
For high frequency digital applications, cross talk is controlled through the placement of ground conductors, however this solution is expensive, reduces manufacturability and decreases reliability. Placing as many ground conductors as possible between adjacent signal conductors reduces cross talk. For example, for a ball grid array (BGA) package, this means surrounding each signal ball with several ground balls, surrounding each signal line with ground lines and surrounding each signal plane with ground planes. The result is a complicated package with many balls, lines and planes. This added complexity increases cost of the electronic package while reducing manufacturability and reliability. The same is true for other types of electronic packages used for digital applications such as pin grid array (PGA) packages, solder column carriers, tape automated bonding (TAB) packages, leaded surface mount packages, and chip-scale packages (CSPs).
For high frequency digital applications, EMI emissions are typically mitigated by shrouding the entire electronic assembly or produce in sheet metal shielding. Sheet metal shielding is bulky and expensive.
There are a variety of RF, microwave and millimeter-wave applications that address the problem of cross talk and EMI emissions with hybrid microcircuit packaging (U.S. Pat. Nos. 5,753,972; 5,736,783; 5,692,298; 5,668,408; 5,465,008; 5,448,826; 5,426,405; 5,285,570; 5,235,300). In hybrid microcircuit packaging approach, the bulk of the electronic package is machined out of a solid block of metal. An intricate array of holes, slots, grooves, and channels must be formed to isolate signal-carrying conductors from each other. After the body of a hybrid microcircuits is machined many other machined components, such as connectors, feed-throughs, interconnect pins and lids are attached. The fabrication and assembly of all these pieces is not well suited for automated manufacturing. Hybrid microcircuits are bulky, expensive and difficult to integrate into larger systems such as printed circuit assemblies. Leaded surface mount packages have been used for high-frequency applications as an alternative to hybrid microcircuits (U.S. Pat. Nos. 5,557,144; 5,522,132; 5,401,912; 5,270,673; 5,160,810; 5,122,621; 5,117,068), however, the leads on these packages are unshielded, causing unwanted EMI emissions.
A variant of solder isolation walls has been reported for environmentally sealing flip-chips (U.S. Pat. Nos. 5,578,874; 5,699,611), however this is limited to only individual integrated circuit chips. Finally, a stacked, three dimensional packaging structure was reported (U.S. Pat. No. 5,706,578), however, it uses expensive machined metal parts to interconnect the layers. What is needed is a simple, low-cost electronic packaging approach that provides integral isolation, electromagnetic shielding and environmental protection while utilizing conventional materials and manufacturing processes.